Cellular radio systems are currently in widespread use throughout the world providing telecommunications to mobile users. In order to meet the capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. At the centre of each cell is a base station through which the mobile stations communicate, typically being equipped with directional antenna arrays arranged in three or six sectored sub-cells where the higher gain of the narrow beamwidth antennas improve the uplink from the lower power mobiles. The distance between the cells is determined such that co-channel interference is maintained at a tolerable level.
Obstacles in a signal path, such as buildings in built-up areas and hills in rural areas, act as signal scatterers and can cause signalling problems. These scattered signals interact and their resultant signal at a receiving antenna is subject to deep and rapid fading and the signal envelope often follows a Rayleigh distribution over short distances, especially in heavily cluttered regions. A receiver moving through this spatially varying field experiences a fading rate which is proportional to its speed and the frequency of the transmission. Since the various components arrive from different directions, there is also a Doppler spread in the received spectrum. All these effects combine so that, in all practical systems, the antenna arrangements must be capable of overcoming at least some of these effects.
A prime consideration in all systems is the cost of the apparatus. A significant cost of any base station is determined by the type of antenna used. Omnidirectional antennas are not dedicated to a particular sector and transmit a 360.degree. azimuthal beam. Such a beam is, typically, narrow in elevation. Omnidirectional antenna installations are therefore simple and cheap to install. An omnidirectional antenna, by its very nature, needs no beam steering and thus with this type of antenna there is no requirement for beam control electronics, further reducing costs. Other types of antennas, for instance, the flat plate antennas, especially of the adaptive variety, have beam steering electronics whereby a beam formed by an array of antenna elements is steered towards, for example, a mobile. Thus sectored antennas are more expensive, not only because the greater number of antennas employed, but also because they require more transceivers per site at initial deployment. For example, a simple omnidirectional site requires only one transceiver whereas a trisectored site will require three transceivers.
When a new cellular radio system is initially deployed, operators are often interested in maximising the range in order to minimise start up costs. Any increase in range means that fewer cells are required to cover a given geographic area, hence reducing the number of base stations and associated infrastructure costs.
The range of the link, either the uplink or the downlink, can be controlled principally in two different ways: by adjusting either the power of the transmitter or the sensitivity of the receiver. On the downlink the most obvious way of increasing the range is to increase the power of the base station transmitter. The output power of a transmitter, however, is constrained to quite a low level to meet national regulations. National regulations, which vary on a country to country basis, set a maximum limit on the effective isotropic radiation power (EIRP) which may be emitted. Accordingly other methods of improving the transmitted gain must be implemented.
One method of improving the receiver sensitivity and to reduce the effect of fading is to include some form of diversity gain. The object of a diverse system is to provide the receiver with more than one path, with the paths being differentiated from each other by some means, e.g. space, angle, frequency or polarisation. The use of these additional paths by the receiver provides the diversity gain. The amount of gain achieved depends upon the type of diversity, number of paths used, and the method of combining the various signals from the several signal paths.
There are several methods of improving the gain for omnidirectional antennas. One method requires the provision of two antennas spaced from each other by typically, 20 wavelengths (which is known as spatial diversity); a second method includes the provision of a linear array of radiating elements vertically stacked one above the other.
The use of two antennas in a spatially diverse system is typically used for repeater stations alongside highways and the like. The two antennas are placed along a line perpendicular to the highway. Omnidirectional antennas can also be grouped in a rectilinear spaced apart relationship, whereby spatial diversity from two antennas can be ensured mobiles. This may not, however, be sufficient to provide the required diversity. The vertical stacking of omnidirectional antennas on the other hand can improve the gain generally and can provide a stronger beam, which is pencil-like in elevation, but does not provide any diversity effects.